Catalytic converters in the exhaust gas of internal combustion engines need a specific minimum temperature (light off temperature) in order to develop their toxic substance converting operation. This temperature is to be reached as rapidly as possible after a cold start. In engines, which are operated with a lean air/fuel mixture (for example, in engines having gasoline-direct injection and nitrogen oxide storage catalytic converters), additional requirements, which are in part changing requirements, are imposed on the catalytic converter temperature, for example, because of a necessary desulfatization of the storage catalytic converter during driving operation. A desulfatization requires, for example, temporarily a higher catalytic converter temperature than during normal operation for the storage of the nitrogen oxides.
Measures are already known for heating catalytic converters. For example, the engine combustion can take place with so rich a mixture that the exhaust gas still contains uncombusted fuel. The supply of secondary air to the exhaust gas causes a reaction-capable mixture to form which heats up the catalytic converter via an exothermal reaction.
Furthermore, the engine combustion can take place with so lean a mixture that the exhaust gas still contains uncombusted oxygen. In this case, a reaction-capable mixture can be generated by the metering of fuel to the exhaust gas.
It is further known to heat up the catalytic converter via the consequences of a deterioration of the efficiency of the engine combustion. A deterioration of efficiency of the engine combustion can, for example, be introduced by a deviation of the ignition time point from the optimal time point. The optimal time point is defined by the maximum efficiency. Because of the reduction of efficiency, the exhaust gas is hotter in comparison to the operation without a loss in efficiency. Accordingly, the exhaust gas develops an increased heating in the catalytic converter.
In engines having gasoline-direct injection, different modes of operation of the engine permit different measures for heating the catalytic converter.
An engine control program is known from U.S. Pat. No. 6,394,063 which controls the switchover between stratified operation and homogeneous operation.
In stratified operation, the engine is operated with an intensely stratified cylinder charge and high air excess in order to achieve the lowest possible fuel consumption. The stratified charge is achieved via a late fuel injection which, in the ideal case, leads to a partitioning of the combustion chamber into two zones: the first zone contains a combustible air/fuel mixture cloud at the spark plug. The first zone is surrounded by the second zone and this second zone comprises an insulating layer of air and residual gas. The potential for optimizing consumption results from the possibility of operating the engine substantially unthrottled while avoiding charge exchange losses. The stratified operation is preferred at comparatively low loads.
At higher load, when the power optimization is primary, the engine is operated with a homogeneous cylinder charge. The homogeneous cylinder charge results from an early fuel injection during the induction operation. As a consequence, a longer time up to the combustion is available for mixture formation. The potential of this operating mode for power optimization results, for example, from utilizing the entire combustion chamber volume for filling with a combustible mixture.
An exhaust-gas composition can be adjusted for heating an NOx-storage catalytic converter in homogeneous operation with this exhaust-gas composition deviating from the stoichiometric exhaust-gas composition.
In gasoline-direct injection engines, the possibility is further provided of targetly injecting fuel into the cylinder in the expansion stroke after the engine combustion when operation is with air excess, that is, preferably, in stratified operation. Here, the after-injected fuel partially reacts with the air excess of the engine combustion partially already in the combustion chamber and partially in the exhaust-gas system. The heat, which is released by the exothermal reaction, heats the catalytic converter.
The task of the invention is to select an optimal heating strategy in each operating state.
The heating of a catalytic converter in accordance with the invention takes place in the exhaust gas of an internal combustion engine which can be operated in different operating modes and wherein at least one of several heating measures can be selected so that at first, for several heating measures, an estimate is made as to whether an individual heating measure can make available the wanted heating effect and that it is further estimated whether an individual heating measure can be carried out in the actual operating state with a view to the exhaust-gas values and the operating mode of the engine necessary for carrying out the heating measure and that that operating state is requested in which the requirements can be best satisfied and that at least one possible heating measure is activated in dependence upon the instantaneous mode of operation. The instantaneous operating state is characterized, for example, by values for the catalytic converter temperature, the vehicle speed and the instantaneous load.
One embodiment is characterized in that a deterioration in the efficiency of the engine combustion takes place as a function of a change of the ignition angle as one measure.
A further measure provides that, as a second measure, a fuel after-injection takes place after the combustion in an engine having gasoline-direct injection.
A further measure provides that the after-injection is combined with stratified operation.
Another measure provides that the air quantity, which is inducted by the engine, is throttled to the extent that the necessary heat flow is achieved at a requested temperature.
A further measure provides that an exhaust-gas composition is adjusted in homogeneous operation for heating an NOx-storage catalytic converter with this exhaust-gas composition deviating from the stoichiometric exhaust-gas composition.
The invention is also directed to an electronic control unit for carrying out the measures and method steps.
The various operating modes of the internal combustion engine having gasoline-direct injection permit different measures for heating the catalytic converter. The allocation of the invention of heating measures and operating modes makes possible an optimization of the heating strategy with a view to the operating state of the vehicle, which, for example, is determined by parameters such as catalytic converter temperature, vehicle speed and torque requirement.
Advantageously, the possible heating effects of various catalytic converter heating measures are estimated and compared to the heating action needed. The heating action needed for heating a catalytic converter leads, for example, to physical requests as to the quantity and the temperature of the exhaust-gas flow which must be made available via the heating measure.
Furthermore, the operating limits for the individual modes of operation are considered. In this way, it is possible to select an optimal heating strategy in each operating mode.
All requirements for active heating measures are formulated as heat flow requests and temperature requests in the exhaust gas. In this way, all requests can be handled in a uniform manner.